The present invention relates generally to x-ray sources and, more particularly, to a surface of an x-ray source employable in diagnostic imaging.
Exemplary diagnostics devices comprise x-ray systems, magnetic resonance (MR) systems, ultrasound systems, computed tomography (CT) systems, positron emission tomography (PET) systems, and other types of imaging systems. Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In connection with the x-ray sources, an x-ray tube in an example comprises an enclosure in which is mounted an anode target adjacent to a cathode. The anode target in an example comprises a disk mounted to a drive shaft for rotation at high speeds. Formed on a face of the target is an annular focal track. The focal track of the anode in an x-ray system is impacted by high energy electrons emitted from the cathode. Exemplary cathodes comprise a tungsten coil, filament, and/or field emitter array. When the high energy electrons from the cathode strike the surface of the focal track of the anode, the electrons are decelerated by the high density focal track. Exemplary materials of the focal track comprise powder metallurgy tungsten or a tungsten-rhenium alloy.
The deceleration of the electrons from the cathode against the surface of the x-ray anode results in the x-ray source. This electron deceleration gives rise to emission of x-rays, secondary electrons, and the generation of heat in a relatively small surface zone, for example, less than 30 microns or micrometers beneath the surface of the focal track. The impinging electrons heat the focal track and in turn the remainder of the target to substantially high temperature during operation. The x-ray anode surface is exposed to significant thermal stress to generate the x-ray radiation, by being struck by the beam of high-energy electrons. The rapid heating of the small, thin surface zone causes a substantial increase in the local target surface temperature, and the generation of enormous thermal stresses that can lead to subsequent cracking of the focal track during thermal cycling, as occurs during repeated x-ray-scanning. The typical cracking that occurs is often called “mud-flat cracking.” During heat up the target surface deforms plastically and during cool down the deformed region is subjected to tensile stresses and subsequent cracking if the tensile stress exceeds the fracture stress of the alloy.
Therefore, it would be desirable to promote an alleviation of thermal stress on the target anode surface with satisfactory performance of the x-ray source.